Commercial and other aircraft are generally equipped with two flight controllers so that the aircraft can be operated either by the pilot or a co-pilot. The aircraft controllers allow manual control of the various control surfaces of the aircraft to control pitch and roll. In a mechanical flight control system, the flight controllers may be linked to the control surfaces via cables and linkages as for example disclosed in U.S. Pat. No. 5,456,428 to Hegg, issued on 10th Oct. 1995. In an electronically controlled aircraft, commonly known as a fly-by-wire aircraft, sidestick motion is transformed into electrical signals which are typically transmitted to the control surfaces via servo motors, actuators or similar devices. An example of a fly-by-wire control system is disclosed in U.S. Pat. No. 4,472,780 to Chenoweth, et al, issued on 18th Sep. 1994.
Controlling an aircraft using electronic rather than mechanical flight controls allows the commands that are executed in the cockpit to be augmented by additional inputs from the flight control computers, and thereby allows for more efficient aircraft operation.
Presently available civil fly-by-wire systems control the aircraft using independently operable pilot and co-pilot sidesticks. There are significant operational and safety benefits of connecting the sidesticks to cause each to move in a pattern that is substantially identical to that of the other. One benefit of linking the sidesticks is to increase the level of safety by providing increased situational awareness for pilots. That is, the pilot and co-pilot will each be able to closely monitor what the other is doing. Thus, in the event that one of them improperly directs the aircraft (i.e. due to sudden illness) the other will immediately recognize the error and be able to correct it. Linking the two sidesticks is also beneficial for pilot training.
Human factor studies have shown that asymmetric forces on a sidestick are needed to provide optimum feel in the lateral roll control movement. This is mainly due to the fact that the arm muscles are stronger when working inboard than outboard. Uncoupled sidesticks have implemented this asymmetry by providing different feel springs for the inboard and outboard direction. Studies have shown that the inboard gradient should be approximately 50 percent higher than the outboard gradient. However, when such sidesticks are coupled together, the asymmetric feel force disappears and the resulting force gradient becomes an average value. Providing a system that will maintain asymmetric forces while ensuring position copy between the sidesticks therefore remains a significant design challenge. A few concepts have been proposed to implement asymmetric roll feel forces, as disclosed in U.S. Pat. No. 5,137,234 and French Patent No. 2,558,136. However, these coupling systems are relatively complex, requiring numerous mechanical components, high tolerances, and which are prone to wear and tear which limits their longevity.